The invention relates generally to an electrochemical cell used as a carbon dioxide sensor, and more particularly to an anion exchange cell for measuring carbon dioxide.
An electrochenmical cell, in simplest terms, consists of an anode (the oxidizing electrode), a cathode (the reducing electrode) and an electrolyte. In order for the electrochemical cell to function, the electrolyte must be compatible with the mechanisms of oxidation and reduction at the electrodes, and it must provide a conductive path for the transport of ionic species between the electrodes.
Electrochemical cells, due to their very high level of sensitivity, are used for measurement in a variety of analytical procedures and many laboratory and process control instruments depend upon the electrochemical cell as a sensor element.
Conventional carbon dioxide sensors use a pH probe to measure the concentration of the hydrogen ion produced when the carbon dioxide is hydrated. One sensor which has widespread acceptance is the Severinghaus carbon dioxide electrode which consists of a standard glass pH electrode covered with a Teflon membrane. Between the glass surface and the membrane exists a thin film of dilute sodium bicarbonate solution. After the carbon dioxide diffuses through the membrane the carbon dioxide becomes equilibrated with the electrolyte solution, the pH of the solution is measured by the glass electrode and it can be interpreted in terms of pCO.sub.2 on the basis of the linear relationship between log pCO.sub.2 and pH as described by the Henderson-Hasselbalch equation.
Although the Severinghaus electrode has gained wide-spread acceptance it is not especially suitable in all instances. This is particularly true where it is desired to measure carbon dioxide content in a respiratory system such as in a diving application or in a biomedical application. Where the pH value is determined in terms of the potential developed by the glass pH electrode, the electrode has a very high internal impedance generally in excess of 200M ohms because the potential is generated by the diffusion of relatively few hydrogen ions across the insulating glass membrane. The glass pH probe is fragile and the electrodes are sensitive to temperature due to changes in properties of the glass membrame. Lastly, most carbon dioxide electrodes have a tendency to drift and frequent calibration is required. The present invention embodies a compact, stable, rugged carbon dioxide sensor suitable for underwater application.
Broadly, the invention is directed to an electrolytic anion exchange sensor based on a diffusion controlled current measurement to determine carbon dioxide content. More particularly, the invention is directed to causing the current to correspond to the diffusion flux of carbon dioxide molecules in an anion exchange cell.
Broadly, my invention comprises an anion exchange cell having a first diffusion membrane permeation selective for CO.sub.2 and an anion exchange membrane spaced apart therefrom to define a cathodic compartment. The cathodic compartment contains a cathode and an electrolyte. A second diffusion membrane is spaced apart from the anion exchange membrane and defines therewith an anodic compartment. The anodic compartment contains an electrolyte and an anode. The carbon dioxide diffused through the first membrane is hydrated forming bicarbonate and hydrogen ions. The bicarbonate ions pass through the anion exchange membrane to the anode. Carbon dioxide is formed and diffuses through the second membrane. The current flowing between the electrodes is measured and this current corresponds to the amount of carbon dioxide in the fluid stream in communication with the cell.